Sending a signal in the form of symbols transmitted at radio frequencies is one way of sending information. Several problems exist with this approach. A wireless communication channel typically experiences fading, i.e. the received signal strength varies with time and the position of the receiver and/or the transmitter. Further, a wireless communication channel often suffers from intersymbol interference, i.e. super-positioning of delayed versions of the transmitted symbol sequence. Intersymbol interference arises, for example, when a receiver picks up delayed versions of a single transmission. Buildings, mountains and other objects create delayed copies of a signal when a transmission reflects off the surface of the object and arrives at the receiver later than a version having fewer or no reflections before arriving at the receiver. The spread in time between the different copies of a signal is called the delay spread. The delay spread results in multiple overlaid copies of the signal with different amplitudes, phases and delays. The multiple copies interfere with the intended signal transmission, becoming noise and causing signal disruption.
Another problem with wireless communication is that the variation in signal strength at a receiver typically requires the system to be designed to transmit with higher power than would be necessary if the signal strength was constant, or if it varied less. This typically reduces the capacity of the system.
S. M. Alamouti [1.2] proposes a method of overcoming this limitation. He provides a two-branch transmit diversity scheme in which two transmit antennas and one receive antenna provide the same diversity as can be achieved with one transmit antenna and two receive antennas. This means that the same reduction in the variation of the quality of the received signal that can be achieved with two receive antennas can instead be realized with two transmit antennas. In the case of a cellular wireless system with base stations and subscriber units, the variability on both the uplink and the downlink can be combated with only multiple antennas at the base station, rather than at the subscriber unit, where it is costly and cumbersome to have multiple antennas.
A problem with the S. M. Alamouti two-branch transmit diversity scheme is that it does not effectively handle intersymbol interference in the channel. When a channel suffers from intersymbol interference, multiple versions of the original symbol sequence are received with different delays making the detection of the symbol sequence more difficult. Intersymbol interference can be caused by multiple propagation paths with different delays or by the use of transmission pulse shaping that extends over more than one symbol interval, or by the receive filter. The transmission pulse shaping and the receive filter is considered to be part of the channel. When there is intersymbol interference in the channel, the S. M. Alamouti scheme loses some of its good properties. More specifically, because of the intersymbol interference in the channel the receiver cannot be realized in the simple form described by S. M. Alamouti. Instead a considerably more complex receiver is be required. This dramatically reduces the usefulness of the scheme for channels with intersymbol interference.
What is needed is a system and method of transmit diversity that enables a transmitter to provide a better signal with less power while still handling intersymbol interference effectively with a relatively simple receiver.